Electron beam apparatus having compensating means for triangular beam distortion



1968 TADAO TAKIZAWA 3,37

ELECTRON BEAM APPARATUS HAVING COMPENSATING MEANS FOR TRIANGULAR BEAMDISTORTION FiledvFeb. 2. 1965 Le mIA 7 I 7 INVENTOR.

United States Patent 3,371,206 ELECTRON BEAM APPARATUS HAVING COM-PENSATING MEANS FOR TRIANGULAR BEAM DISTORTION Tatiao Takizawa,Akishima-shi, Tokyo, Japan, assignor to Nihon Denshi Kabushiki Kaisha,Tokyo, Japan, a corporation of Japan Filed Feb. 2, 1965, Ser. No.429,803 Claims priority, application Japan, Feb. 4, 1964, 39/5,521 4Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE An apparatus forirradiating a workpiece with an electron beam having a beam source andmeans for focusing the beam and also having a magnetic deflection systemand a compensator positioned between the deflection system and focusingmeans to compensate for triangular image distortion introduced by thedeflection system.

This invention relates to an improved method and apparatus for treatinga workpiece with a charged electron beam.

In the conventional electron beam apparatus, the beam is focused ontothe surface of the workpiece by a magnetic lens and also passes througha magnetic deflection field which is a variable magnetic field that isdisposed to cause the beam to bend so as to change its position alongthe workpiece surface. The problem generally arises that When anelectron beam is deflected by such a field, it is undesirably distortedresulting in astigmatism and coma of the focused beam on the workpiecesurface. The distorted image on the workpiece in gen eral takes on astar-like pattern and accordingly the area of the image becomes largerand the energy density of the beam is lowered.

The above-mentioned aberrations seriously limit the use of chargedelectron beams for such applications as boring, milling, melding andwelding.

The usual steps for eliminating such image distortion in conventionalapparatus has been to position a slit member under the deflector. Thispractice provides a means for obtaining a circular beam spot on thesurface of the material by eliminating the outer part of the distortedstar-like pattern. However, by doing this, the energy density and theefliciency of the application are inevitably lowered.

The present invention provides a method and apparatus for making acircular beam spot or image on the surface of the material duringmagnetic deflection without lowering the energy density of the beam. Ourapparatus is automatically controlled in conjunction with the deflector.Thus, our invention overcomes the difliculties of beam distortion.

Our invention completely eliminates beam aberration by a compensatingdevice, such as is depicted by FIG- URE 4 of the drawings.

In the drawings, I have illustrated a preferred embodiment of myinvention in which:

FIGURE 1 is a schematic diagram showing the prin- 3,321,296 PatentedFeb. 27, 1968 ciples of electron beam deflection as accomplished byconventional means;

FIGURE 1A is a diagram showing a deflection lens deflecting an advancingelectron beam and casting its image on a plane;

FIGURE 2 is an illustrative plan view of electron beam images from thedevice of FIGURE 1 as projected on a workpiece during four positions ofthe beam;

FIGURE 2A is an illustrative plan view of an electron beam imagedeflected onto an ordinate x and an ab scissa y;

FIGURE 3 is a schematic diagram similar to that of FIGURE 1 but showingthe application of the apparatus of the present invention; and

FIGURE 4 is a plan view of a compensator device that embodies theprinciples of the present invention.

Referring to FIGURE 1, there is illustrated the principles of aconventional charged beam apparatus. The apparatus of FIGURE 1 isdescribed in advance of a detailed description of the present inventionso that the principles of operation of the present invention are morefully understood.

An electron beam EB is emitted from an electron gun 1 and is focused bya condenser lens 2, and then deflected by a deflector lens 3 prior toirradiating the workpiece 4 that is being treated. The said beam iscontrolled to a fixed angle with respect to the axis of the beam, suchas angle 6 0 or 0 as shown by FIGURE 1. The angle of deflection isdependent upon the intensity of the magnetic field. The stronger theintensity of the magnetic field, the more the deflection angle of thebeam will be.

Additionally, the image of the beam that is formed on the workpiecevaries in its shape according to the angle of the beam or the intensityof the magnetic field posed by the deflection device.

In FIGURE 2, (a) shows the image when the beam irradiates the workpieceperpendicularly with respect to its surface. (b), (c), and (d) of FIGURE2 show the images formed on the surface of the material or the workpiecewhen the beam is progressively deflected through angles, such as angles0 0 and 0 As the intensity of the deflection field increases, the imageis gradually distorted and finally appears as a star-like pattern asshown by (d) of FIGURE 2.

In FIGURE 3, an electron gun 5 is shown which is identical to thatconventionally employed. This gun is composed of a cathode 6, an anode7, and a grid 8. The electron beam EB focused by the focusing lens 9 isatfected by the magnetic field of a compensator 10 which is positionedbetween the focusing lens 9 and the deflector 11. This compensator isoperated by means of an electric source 12, and the deflector isprovided with an electric source 13.

The compensator is energized in accordance with the intensity of theelectric current used in the deflector and is preset or adjusted so thatit effects an electron beam image that is conversely symmetrical to theone formed by the deflector.

The value of the resistance to electric current flow in the compensatoris adjusted so that current flow will not take place when the deflectorhas no effect on the advancing beam.

FIGURE 4 illustrates a compensator which embodies the principle of thepresent invention in obtaining a conversely symmetrical image to thedistorted image that is created by the deflection lens. In thisembodiment, coils 14a, 14b, 14c, 14a, 1412 and 14 are connected inseries to an electric source 12 and are arranged symmetrically aroundthe beam passage so that their respective polarities oppose one another.With this arrangement, a substantially triangular magnetic field iscreated (see dotted outline in FIG. 4) which is imposed by the magneticinteraction between the poles.

As shown above, magnetic deflection of the electron beam creates asubstantially triangular shaped image. Accordingly, the compensator isconstructed in such a manner that its magnetic influence shapes theelectron beam so that it is conversely symmetrically triangular to theshape created by the deflector distortion. This results in a beam spotor image on the face of the material that is substantially circularshaped because of the two different influences of the deflector and thecompensator lenses.

The beam image on the workpiece which is distorted when deflected by anordinary deflector as shown in FIG- URE 1A is illustrated in FIGURE 2Ain conjunction with an ordinate x and an abscissa y. The junction of xand y intersects the nondistortecl center point of the image. Therespective relationship between a given deflection angle and x and y areillustrated by the following equation:

where h and f are the abbreviations of function.

In greater detail, the above distortion relationships are given by thefollowing general distortion formulae:

where:

ro=the beam radius on the principal plane of the deflector rs=the beamradius on the perpendicular plane l=the distance from the deflectorsprincipal plane 0=the deflection angle q=defocusing, i.e. the distancein centimeters of the shifting of the focusing point by the deflectionof the beam 2b =a half width along the z axis (the direction of theincident beam).

In my investigations, I presumed that distortion was caused by bothelements of 70 sin 2X in the formula introduced by x and 01 0 cos 2X inthe formula introduced by y.

When employing a cylindrically coordinated six-pole magnetic deflectorsuch as that depicted by FIGURE 4, the following correlation ispossible.

where r the beam radius on the principal plane of the deflector, fi theangle between the point on the crosssection of the beam on the principalplane of the deflector and the y axis, the cylindrical beam passingthrough is influenced as shown by the following ratio:

where Is is distance from the six pole magnetic field to the image planeand A is a constant.

Accordingly, distortion for any given angle of deflection may beeliminated by selecting and properly determining the constant C inFormula 3 above. The constant C in actuality involves such factors aselectric current and mechanical factors. However, if the mechanicalfactors are constant, then C is substantially proportional to theelectric current.

The electric current flow to the compensator is not proportional to theelectric current flow to the deflector. However, as 'shown above, arelationship exists between the electric current flow into thecompensator and into the deflector.

Consequently, current flow into compensator 10 may be regulated by anyconventional current flow apparatus such as a rheostat and a currentflow established for any given current flow into deflector 11 tocompensate for any given distortion for any given angle of deflection.

Thus, by employing our invention, the resulting electron beam image onthe surface of the material is circular in shape or identical to theshape created when the beam is perpendicularly irradiated with respectto the surface of the material.

Although in the present invention the embodiment of FIGURE 4 shows asix-pole compensator utilized for compensating beam distortion, it ispossible to employ a compensator having more than six poles and,additionally, it is possible to employ an electrostatic field in placeof the magnetic fields and a combination of magnetic and electrostaticfield can be used without changing the principles of the presentinvention.

The following data is typical of an operation of the above describedapparatus:

Accelerating voltage of the electron gun25KV.

Deflection angle of the beam in respect to the beam axis Currentsupplied to the deflector 110'.0l8 ampere.

Coil turns in the deflector 11--10,500.

Number of poles of deflector 112.

Current supplied to the compensator 10-0.3 ampere.

Number of coil turns in compensator 10150.

Number of poles in compensator 106.

While I have described the presently preferred embodiments of myinvention, it is to be understood that it may be otherwise embodiedwithin the scope of the following claims.

I claim:

1. An apparatus for irradiating the surface of a material with anelectron beam comprising:

(a) an electron beam source;

(b) focus means for directing the beam onto the surface of the material;

(c) magnetic deflection means for subjecting the electron beam tomagnetic influences to change the position of the beam on the material,said deflection means causing the beam to be distorted in a substantially triangular manner; and

(d) compensator means having at least six poles and positioned betweenthe focus means and the deflection means, said poles shaping the beam ina complementary manner to the distortion created by the deflection meansby means of at least one influencing factor selected from the groupconsisting of magnetic fields and electrostatic fields.

2. An apparatus as set forth in claim 1 wherein said poles influencesaid beam by means of a magnetic field and wherein said poles arearranged so that each opposite and adjacent pole is antipodal.

3. An apparatus as set forth in claim 1 wherein said poles influencesaid beam by means of an electrostatic field and wherein said poles arearranged so that each opposite and adjacent pole is antipodal.

4. An apparatus as set forth in claim 1 including means for supplying anelectric current to the compensator means that is substantiallyproportional to the current supplied to the deflection means so that thecompensator means will eifect the field at substantially the correctstrength to create the complementary influence.

References Cited UNITED STATES PATENTS RALPH G, NILSON, PrimaryExaminer. 10 A. L. BIRCH, Assistant Examiner.

